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Several towns issued evacuation advisories to residents nearest the coast. A swelling of 10 centimeters (2.5 inches) was observed in the town of Erimo on Hokkaido island, while no major changes were reported elsewhere.

The Japan Meteorological Agency said the tremor was 6.8 magnitude and forecast a tsunami of up to half-meter (12 inches) could hit Hokkaido island's central and eastern Pacific coast, and Aomori and Iwate prefectures in the north of Japan's main island.

Iwate prefecture, or state, was heavily damaged by last year's earthquake and tsunami.

Thousands of aftershocks have shaken the region since then, nearly all of them of minor or moderate strength. The magnitude 9.0 earthquake and tsunami on March 11, 2011, ravaged many Japanese northern coastal towns, leaving some 19,000 people dead or missing.

Wednesday's tremor was centered about 146 miles (235 kilometers) south of Kushiro, Japan, on Hokkaido island.

It was a relatively shallow 6 miles (10 kilometers) below the sea surface.

The quake was not felt in Tokyo, and there was no threat of a Pacific-wide tsunami.